Fluidized bed coating methods have heretofore been widely investigated for depositing various coatings onto numerous core particles. In the nuclear reactor field these coaters have been utilized to deposit such coatings as pyrolytic carbon onto the various nuclear fuel particles; viz, oxides, carbides, and nitrides of the actinide metals thorium, uranium, and plutonium. Similarly, compounds to the Group IV metals, such as titanium, zirconium, and hafnium, have been successfully coated with pyrolytic carbon for various applications.
Nuclear fuel particles typically have a density in the range 8 to 11 g/cc and a diameter 200 to 600 microns. These particles are sufficiently large and have densities such that during coating they remain as separate distinct particles. For gas-cooled reactors, such as Ft. St. Vrain HTGR one fuel concept comprised a pyrolytically coated, low density, small diameter UC.sub.2 + fuel particle. Attempts to coat these small, low density particles have received little success using known prior art coating techniques. Apparently, when the hydrocarbon coating gas begins to decompose, the fluidizing particles and the wall of the coater become "tacky" such that the individual particles stick together and to the vessel walls.